Silicon crystals, as used herein, are cylindrical blocks of pure silicon that semiconductor device manufacturers use to form semiconductor wafers. The manufacturers form the wafers by very carefully and precisely slicing the silicon crystal to produce a flat, mostly circular, thin piece of silicon. A common technique for growing a silicon crystal is known as the Czochralski technique. The Czochralski technique places a silicon seed in a susceptor or heated container filled with molten silicon. The technique is to withdraw the seed from the molten silicon while rotating the seed and susceptor and cooling the silicon that attaches to the seed. The combination of slowly withdrawing the seed, rotating the seed and susceptor, and cooling the silicon that adheres to the seed controllably grows the silicon crystal and permits precise determination of the crystal's diameter and length.
A problem in the Czochralski technique is to control oxygen concentrations in the silicon crystal. A major problem of having oxygen in the silicon crystal is known as oxygen-induced stacking fault or OISF. OISF occurs due to oxygen micro-defects that occur in the crystal. There are some beneficial effects of having a small amount of oxygen in the silicon crystal. For example, a controlled amount of precipitation of small oxygen clusters has the beneficial, metal-gettering effect of forming dislocation angles in the crystal lattice where the molecular oxygen precipitates. There are, however, difficulties in achieving an optional combination of the benefits of oxygen precipitation and freedom from OISF. These difficulties are compounded by other requirements in manufacturing silicon wafers, such as that the silicon be dislocation-free and that the resistivity, resistivity gradients, oxygen concentration, oxygen gradients, and the crystal diameter be within certain ranges. Therefore, deriving the beneficial effects from a small amount of oxygen, however, is quickly overcome by the detrimental effects that OISF causes in the crystal as well as the necessary monitoring and control of process parameters.
The problems that OISF and too much oxygen precipitation cause in the resulting electronic circuit include interruptions in electronic device operation as well as changes in signal properties. Because of these and other problems, generally it is necessary that silicon crystals be free of oxygen micro-defects, including oxygen precipitation. As a result of both OISF and oxygen precipitation, it is often necessary to discard as much as two-thirds of a silicon crystal. That is, in the fabrication of a silicon crystal, OISF may cause a loss of as much as one-third of the top portion of the silicon ingot. Oxygen precipitation defects, in addition, may cause a loss of as much as the bottom third of the silicon crystal. Since the typical preparation time for silicon crystal can be 25 hours, if as much as 50% or more of the silicon crystal must be discarded, or at best recycled, a significant loss in efficiency and productivity occurs using known techniques that employ the existing Czochralski silicon crystal growing techniques.
In silicon crystals grown by the Czochralski technique, for example, the silicon melt temperature and the morphology of the interface between the molten silicon and the growing crystal are important considerations that determine the crystal purity. Although it is well-known that OISF can occur in silicon crystals grown by the Czochralski technique, there is no known method or system that controls OISF in single silicon crystals.
The problem with OISF is that it causes circuit malfunctions in semiconductor wafers formed from silicon crystals that have OISF. U.S. patent application Ser. No. (TI-15889), by W. Bell, at al, entitled "OISF Control in Czochralski-Grown Silicon Crystals" (hereinafter W. Bell), assigned to Texas Instruments Incorporated and filed on May 6, 1994 addresses the oxygen precipitation problem and is here incorporated by reference.